1. Field of the Invention
The invention relates to a hydraulic transaxle incorporating a hydraulic motor, and to a vehicle having front and rear hydraulic transaxles having respective hydraulic motors.
2. Related Art
Conventionally, there is a well-known working vehicle having front and rear transaxles incorporating respective hydraulic motors, which are supplied with hydraulic fluid from a pump unit separated from the transaxles, as disclosed in U.S. Pat. No. 6,732,828.
Each of the hydraulic motors is set into either a variable displacement type or a fixed displacement type, corresponding to a steering style of the vehicle comprising the transaxles, such as an Ackerman type or an articulate type.
If a vehicle has the Ackerman type steering mechanism, i.e., has steerable wheels and unsteerable wheels, the steerable wheels must be rotated faster than the unsteerable wheels during turning of the vehicle. If a vehicle has the articulated type steering mechanism and either front or rear wheels become more distant from the turning center of the vehicle than the other rear or front wheels during turning of the vehicle, the wheels becoming more distant from the turning center are rotated faster than the other wheels during turning of the vehicle. The difference of wheel speed between the front and rear wheels during turning of the vehicle prevents dragging of the wheels. Correspondingly, the variable displacement hydraulic motor of the two motors in the respective transaxles must change its displacement.
Conventionally, the above-mentioned difference of displacement style between the hydraulic motors has prevented standardization of the front and rear transaxles. Namely, two types of hydraulic transaxles: one incorporating a variable displacement hydraulic motor; and the other incorporating a fixed displacement hydraulic motor, must be prepared for suiting any of vehicles having various steering systems, thereby increasing a burden on makers in management of parts and production.
In the view of standardization, it is preferable that the transaxles have standardized deceleration gearings and hydraulic motors. However, standardization of the deceleration gearing has the following problem in the combination of the transaxles incorporating respective hydraulic motors, one of which is a variable displacement type, and the other of which is a fixed displacement type.
The variable displacement hydraulic motor of the transaxle required to accelerate during turning of the vehicle usually applies its maximum displacement for straight traveling of the vehicle, and its minimum displacement for turning of the vehicle. In the condition of both the transaxles having the same gear reduction ratio due to the standardization of the deceleration gearing, the maximum displacement of the variable displacement hydraulic motor is equalized to the fixed displacement of the other hydraulic motor. In other words, the minimum motor speed of the variable displacement hydraulic motor is equalized to the motor speed of the fixed displacement hydraulic motor during straight traveling of the vehicle. However, the deceleration gearing is usually standardized corresponding to the fixed displacement hydraulic motor. In such a situation, acceleration of the variable displacement hydraulic motor during turning of the vehicle means unexpected acceleration of the deceleration gearing of the corresponding transaxle such as to generate unexpectedly large noise.